Wafer temperature sensing methods and related semiconductor wafer

ABSTRACT

A method includes measuring a first voltage across a test diode on a semiconductor wafer while injecting a first current into the test diode, measuring a second voltage across the test diode while injecting a second current into the test diode, and determining temperature of a region proximate the test diode according to difference between the first voltage and the second voltage.

BACKGROUND

The semiconductor industry has experienced rapid growth due toimprovements in the integration density of a variety of electroniccomponents (e.g., transistors, diodes, resistors, capacitors, etc.). Forthe most part, this improvement in integration density has come fromshrinking the semiconductor process node (e.g., shrinking the processnode towards the sub-20 nm node). As device dimensions shrink, voltagenodes also shrink, with core device voltages trending toward less than 1Volt, and input/output (I/O) device voltages under 2 Volts. Temperaturevariation of device parameters, such as transistor threshold voltage, isa concern as voltage nodes shrink. For example, transistor thresholdvoltage may vary on the order of single millivolts per degree Celsius(e.g., −4 mV/° C. to −2 mV/° C. depending on doping level). Integratedcircuits (ICs) are expected to operate in large temperature ranges(e.g., 0° C. to 70° C. for “commercial” ICs), which correspond to largetemperature variations (e.g., ±140 mV) that may be on the same order ofmagnitude as the device parameter (e.g., transistor threshold voltage of450 millivolts). Characterization of circuit performance for temperaturevariation, therefore, is increasingly important.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a circuit diagram of a test system in accordance withvarious embodiments of the present disclosure;

FIG. 2 shows a circuit diagram of a current source in accordance withvarious embodiments of the present disclosure;

FIG. 3 shows a diagram of a semiconductor wafer in accordance withvarious embodiments of the present disclosure;

FIG. 4 shows a zoomed-in view of an integrated circuit die with thermalsensor location candidates in accordance with various embodiments of thepresent disclosure;

FIG. 5 is a flowchart diagram showing a process for sensing temperaturein a semiconductor wafer in accordance with various embodiments of thepresent disclosure;

FIG. 6 is a flowchart diagram showing a test process using the thermalsensor in accordance with various embodiments of the present disclosure;and

FIG. 7 illustrates a test system in which embodiments of the inventionmay be employed.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present disclosureprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the disclosedsubject matter, and do not limit the scope of the different embodiments.

Embodiments will be described with respect to a specific context, namelyan integrated thermal sensor, and the like. Other embodiments may alsobe applied, however, to other types of thermal test circuits.

Throughout the various figures and discussion, like reference numbersrefer to like components. Also, although singular components may bedepicted throughout some of the figures, this is for simplicity ofillustration and ease of discussion. A person having ordinary skill inthe art will readily appreciate that such discussion and depiction canbe and usually is applicable for many components within a structure.

IC performance is typically characterized for process, voltage, andtemperature variation. Temperature variation characterization may beperformed by attaching a wafer to a thermal chuck, which heats and/orcools the wafer to specific temperatures for circuit performancecharacterization. Absolute temperature of the wafer heated by thethermal chuck is not perfectly uniform. For example, the thermal chuckmay be set to heat the wafer to 50° C., but the wafer may have regionswith temperatures ranging from 46° C. to 50° C. Thus, a temperatureprofile of the thermal chuck is typically obtained through use of athermal couple prior to testing the wafer for circuit performance undertemperature variation. The temperature profile may be obtained byheating/cooling the wafer to various temperatures (e.g., −25° C., 0° C.,25° C., 50° C., and 70° C.), allowing temperature of the wafer tostabilize for 30 minutes to one hour, then obtaining a number oftemperature data points at positions distributed over the surface of thewafer. For example, five data points spread over the surface of thewafer may be obtained for each temperature.

Three problems arise when using the thermal profile obtained through thethermal couple. First, the thermal couple is not able to provide anaccurate reading of the temperatures at the various data points. Forexample, a typical thermal couple has temperature error in a range ofabout ±1-2° C. Second, during test, the wafer temperature must beallowed to stabilize for 30 minutes to one hour before circuitperformance at the test temperature can be characterized. This greatlyinhibits throughput, as each test temperature requires the stabilizationperiod, and anywhere from 5 to 8 (or more) test temperatures may becharacterized. Finally, distance from a temperature sensitive circuitunder test to the nearest available data point on the thermal profilemay be large, such that absolute temperature at the location of thecircuit under test is hard to determine with any confidence.

In the following disclosure, a novel integrated thermal sensor andtemperature characterization method are introduced. Through use of anintegrated diode or bipolar junction transistor and four test contactpads, the thermal sensor can be placed anywhere on the wafer, has betteraccuracy than the thermal probe, allows for rapid determination of localtemperature without need for stabilization, and saves layout space. Thethermal sensor is small, and can be placed on a test line to obtainwafer temperature. The thermal sensor also relaxes accuracy and mismatchrequirements on a trimming circuit that is common involtage-to-frequency (V2F) and sigma-delta analog-to-digital converter(Σ-Δ ADC) sensing circuits.

A circuit diagram of a test system 10 in accordance with variousembodiments of the present disclosure is shown in FIG. 1. A thermalsensor 100 receives current input from a current source 110, and outputsvoltage to a voltage meter 120. The voltage outputted per unit currentvaries approximately linearly with temperature. The thermal sensor 100is formed on a semiconductor wafer, for example. In some embodiments,the thermal sensor 100 is formed in a semiconductor package. The currentsource 110 and the voltage meter 120 may be external to thesemiconductor wafer. Temperature of the semiconductor wafer near thethermal sensor 100 can be determined by applying two different currentsto the thermal sensor 100 through the current source 110, and readingoutput voltages of the thermal sensor 100 corresponding to the twodifferent currents through the voltage meter 120.

Placement of the thermal sensor 100 can be highly flexible, such aswithin an integrated circuit die, in a test line of the semiconductorwafer, in a scribe line, or the like. In a single semiconductor wafer,more than one of the thermal sensors 100 may be placed at locationsthroughout the semiconductor wafer. The thermal sensors 100 may beplaced at regular intervals over the area of the semiconductor wafer toobtain a thermal profile thereof, for example. The thermal sensors 100may also be placed proximate temperature sensitive circuits to gethighly accurate temperature readings at sensitive areas of eachintegrated circuit die.

Current pads 101, 102 of the thermal sensor 100 are electricallyconnected to terminals of the current source 110 and terminals of adiode circuit 105 of the thermal sensor 100. The current pads 101, 102may be metallic contact pads formed over the semiconductor wafer, andmay contact the terminals of the diode circuit 105 through multiplemetallization layers and contact vias, for example. The current pad 101contacts an anode electrode of the diode circuit 105, and the currentpad 102 contacts a cathode electrode of the diode circuit 105.

Voltage pads 103, 104 of the thermal sensor 100 are electricallyconnected to terminals of the voltage meter 120 and terminals of thediode circuit 105 of the thermal sensor 100. The voltage pads 103, 104may be metallic contact pads formed over the semiconductor wafer, andmay contact the terminals of the diode circuit 105 through multiplemetallization layers and contact vias, for example. The voltage pad 103contacts the anode electrode of the diode circuit 105, and the voltagepad 104 contacts the cathode electrode of the diode circuit 105.

The diode circuit 105 may be a PN-junction formed in the semiconductorwafer, a bipolar junction transistor (BJT) in diode-connectedconfiguration, or the like. The diode circuit 105 has a well-definedrelationship between change in voltage (ΔV) to temperature. For example,the diode circuit 105 may be a BJT having a correlation between changein base-emitter voltage (ΔV_(BE)) and temperature. A temperaturedependence relationship of the diode circuit 105 may be given by thefollowing equation:

$\begin{matrix}{{\Delta\; V_{BE}} = {\frac{KT}{q} \times {\ln( \frac{I_{C\; 1}}{I_{C\; 2}} )}}} & (1)\end{matrix}$

where V_(BE) is bandgap voltage of the diode circuit 105, K isBoltzmann's constant, T is temperature in kelvins, q is charge on anelectron, and I_(C1) and I_(C2) are two different currents. Solving fortemperature, the following equation is obtained from equation (1):

$\begin{matrix}{T = \frac{\Delta\; V_{BE} \times q}{K\;{\ln(N)}}} & (2)\end{matrix}$

where N is the ratio of I_(C1) to I_(c2) (e.g., 2). For a ratio N of 2,a change in temperature T of 1° C. relates to a corresponding change inΔV_(BE) of 59.7 μV, whereas for N=10, the corresponding change inΔV_(BE) for each degree of temperature T increases to 198 μV, or about0.2 mV.

In practical situations, the thermal sensor 100 may not have idealperformance over a target temperature range. For example, ΔV_(BE) maynot be linearly correlated with temperature. For example, atenvironmental temperature of 0 degrees, the thermal sensor 100 mayexhibit ΔV_(BE) corresponding to 2 degrees (error=2 degrees), and atenvironmental temperature of 100 degrees, the thermal sensor 100 mayexhibit ΔV_(BE) corresponding to 98.5 degrees (error=−1.5 degrees). Theerror may not be linearly correlated over temperature, either. Thus, thethermal sensor 100 may be calibrated through two-point calibration, forexample. Multiple-point calibration may also be used to obtain a curve,and a linear approximation of the curve may be obtained throughinterpolation. Calibration parameters correlating ΔV_(BE) to temperaturemay be stored in a lookup table, or a calibration equation may bederived so that the temperature reading outputted by the thermal sensor100 may be calibrated on the fly. Simulations have shown that theabsolute temperature read by the thermal sensor 100 after calibrationmay achieve as good as ±0.75 degrees Celsius confidence.

The current source 110 may be external to the semiconductor wafer (e.g.,a DC current supply), and is capable of inputting at least two differentcurrents (e.g., the currents I_(C1) and I_(C2) in equation (1)) to thediode circuit 105 through the current pads 101, 102. For example, thecurrent source 110 may input a first current to the diode circuit 105,then input a second current to the diode circuit 105 equal to double thefirst current. In general, the second current may be any multiple of thefirst current, and is not limited to integer multiples. And, order ofinputting the first current and the second current may be reversed. Asmentioned above, more than two currents may be inputted to determine thetemperature of the diode circuit 105.

The voltage meter 120 may also be external to the semiconductor wafer,and is capable of sensing the bandgap voltage of the diode circuit 105through the voltage pads 103, 104. The voltage meter 120 may draw littleto no current when measuring the voltage across the voltage pads 103,104, so as not to affect current flow through the diode circuit 105 setup by the current source 110. The voltage meter 120 may detect a firstvoltage (e.g., V_(BE1)) while the first current is inputted to the diodecircuit 105 by the current source 110, and may further detect a secondvoltage (e.g., V_(BE2)) while the second current is inputted to thediode circuit 105 by the current source 110. Then, ΔV_(BE) can becalculated as V_(BE2)−V_(BE1).

A circuit diagram of one example of the current source 110 is shown inFIG. 2. A first current supply 111 is electrically connected across thecurrent pads 101, 102 through a first switch 113. The first currentsupply 111 has a first terminal electrically connected to the currentpad 101. The first switch 113 has a first terminal electricallyconnected to a second terminal of the first current supply 111, and asecond terminal electrically connected to the current pad 102. The firstswitch 113 may be a MOSFET switch, a pass gate, or the like, and iscontrollable by an electrical input (e.g., a voltage) applied at acontrol terminal thereof. The first current supply 111 outputs areference current I_(REF), which may be a highly accurate current, suchas a bandgap reference current.

A second current supply 112 is electrically connected across the currentpads 101, 102 through a second switch 114. The second current supply 112has a first terminal electrically connected to the current pad 101. Thesecond switch 114 has a first terminal electrically connected to asecond terminal of the second current supply 112, and a second terminalelectrically connected to the current pad 102. The second switch 114 maybe a MOSFET switch, a pass gate, or the like, and is controllable by anelectrical input (e.g., a voltage) applied at a control terminalthereof. The second current supply 112 outputs a multiple of thereference current I_(REF) (e.g., N×I_(REF)), and may also be a highlyaccurate current, such as a bandgap reference current. The multiple Nmay be an integer multiple, for example, such as 2 or 10.

In some embodiments, the first switch 113 may be omitted or normally on,so that the first current supplied to the thermal sensor 100 is I_(REF),and the second current supplied when the second switch 114 is turned onis (N+1)×I_(REF). For the example where N is 2, the first and secondcurrent supplies 111, 112 may be identical by using the switching schemejust described.

A semiconductor wafer 300 (or simply “wafer 300”) in accordance withvarious embodiments of the present disclosure is shown in FIG. 3.Integrated circuit dies 310 and 320 (or simply “dies 310 and 320”) areformed in and on the semiconductor wafer 300, and may include activecircuits, passive circuits, and interconnect structures. Atemperature-sensitive circuit 311 is formed in the integrated circuitdie 310. Only two dies are shown in FIG. 3. A typical semiconductorwafer will be optimally filled with integrated circuit dies depending ondimensions of the dies. Horizontal scribe lines 331 and 332 and verticalscribe lines 333, 334 run between rows and columns of dies,respectively, and serve multiple purposes in fabrication of theintegrated circuit dies. The scribe lines 331-334 physically isolateindividual dies from each other, and provide a guideline for a diamondsaw during singulation. Prior to singulation, the scribe lines 331-334may also be used for placement of test circuits for testing electricaland functional characteristics of the dies 310, 320. The test circuitsmay also be placed within the dies 310, 320.

A zoomed-in view of the integrated circuit die 310 with thermal sensorlocation candidates 411-414, 431-434 in accordance with variousembodiments of the present disclosure is shown in FIG. 4. The thermalsensor 100 of FIGS. 1 and 2 may be placed at various locations on thewafer 300. Candidate locations 411-414 reside within the integratedcircuit die 310, whereas candidate locations 431-434 reside within thescribe lines 331-334, respectively. The candidate location 411 islocated above and proximal to the temperature-sensitive circuit 311. Thecandidate location 412 is located below and proximal to thetemperature-sensitive circuit 311. The candidate location 413 is locatedto the left of and proximal to the temperature-sensitive circuit 311.The candidate location 414 is located to the right of and proximal tothe temperature-sensitive circuit 311. Another candidate location 415 iswithin the die 310, but relatively not proximal to thetemperature-sensitive circuit 311. The candidate location 415 may beuseful when insufficient space exists surrounding thetemperature-sensitive circuit 311, for example.

The candidate locations 431-434 are located within the scribe lines331-334. The candidate location 431 is located on the scribe line 331,which resides above the die 310, and is relatively not proximal thetemperature-sensitive circuit 311 shown in FIG. 4. The candidatelocation 432 is located on the scribe line 332 below the die 310, and isrelatively proximal the temperature-sensitive circuit 311. The candidatelocation 433 is located on the scribe line 333, and is both to the leftof and relatively proximal to the temperature-sensitive circuit 311. Thecandidate location 434 is located on the scribe line 334, and is both tothe right of and relatively proximal to the temperature-sensitivecircuit 311. Embodiments where the temperature-sensitive circuit 311 isproximal to one, two, three or all four of the scribe lines 331-334, andplacement of the thermal sensor 100 in candidate positions proximaland/or relatively not proximal to the temperature-sensitive circuit 311are contemplated herein. Embodiments in which multipletemperature-sensitive circuits are located within the die 310, and atleast one thermal sensor is formed proximate each temperature-sensitivecircuit are also contemplated herein.

Advantageously, the candidate positions 411-414 are very close to thetemperature-sensitive circuit 311, which ensures good knowledge oftemperature of the wafer 300 in the region in which thetemperature-sensitive circuit 311 is located. Locating the thermalsensor 100 in the candidate positions 432-434 saves area, while alsoproviding relatively proximal temperature readings relative to theregion in which the temperature-sensitive circuit 311 is located. Thethermal sensor 100 is also fast, and highly accurate, allowing forgreater speed and flexibility when performing temperature-dependent testprocedures on the integrated circuit die 310 or individual blocksthereof (e.g., the temperature-sensitive circuit 311).

A process 50 for sensing temperature in a semiconductor wafer inaccordance with various embodiments of the present disclosure is shownin FIG. 5. The process 50 is described in terms of the test system 10shown in FIGS. 1-4. To determine temperature of the semiconductor waferusing the test system 10, a first current is inputted to the test diode105 in block 500. The test diode 105 is located in the semiconductorwafer (e.g., near a temperature-sensitive circuit, on a test line, on ascribe line, or the like). Block 500 may be performed by the currentsource 110 by activating either of the current supplies 111, 112 toinject current into the test diode 105. The injection may be controlledby an operator, and/or by automatic test equipment including the currentsource 110 and a controller, for example. The first current inputted inblock 500 may be on the order of microamperes, such as in a range ofabout 2 microamperes to 20 microamperes. Other ranges for the firstcurrent are also contemplated herein.

The first current injected in block 500 by the current source 110 setsup a first voltage across the test diode 105, and in block 502, thefirst voltage is measured as the first current is flowing through thetest diode 105. The first voltage may be read by the voltage meter 120.The first current may be allowed to stabilize before the first voltageis read. The first voltage is a bandgap voltage of the test diode 105 insome embodiments. The voltage meter 120 may read the first voltage bydetecting and quantizing a potential difference across the voltage pads103, 104. The first voltage read out by the voltage meter 120 may bestored for use in calculations (e.g., in determining the temperaturenear the thermal sensor 100).

After completing block 502, a second current is inputted to the testdiode 105 by the current source 110 in block 504. The first current maybe turned off (e.g., through operation of the first switch 113) prior toactivating the second current. The second current may be inputted to thetest diode 105 by the second current supply 112 controlled by the secondswitch 114. The second current may be a multiple of the first current,or the second current may be a fraction of the first current. Magnitudeof the second current may be on the order of microamperes, such as in arange of about 2 microamperes to about 20 microamperes, for example.Other ranges for the second current are also contemplated herein. Theratio of the second current to the first current may be 1/10, 1/2, 2,10, or the like.

While keeping the second current flowing through the test diode 105, asecond voltage of the test diode 105 may be measured by the voltagemeter in block 506. The second current injected in block 504 by thecurrent source 110 sets up the second voltage across the test diode 105.The second voltage may be read by the voltage meter 120. The secondcurrent may be allowed to stabilize before the second voltage is read.The second voltage is a bandgap voltage of the test diode 105 in someembodiments. The voltage meter 120 may read the second voltage bydetecting and quantizing a potential difference across the voltage pads103, 104. The second voltage read out by the voltage meter 120 may bestored for use in calculations (e.g., in determining the temperaturenear the thermal sensor 100).

Knowing the first voltage corresponding to the first current, and thesecond voltage corresponding to the second current, the temperature ofthe region in which the thermal sensor 100 is located can be determinedin block 508. Equation (2) above can be used to determine thetemperature according to the difference of the first voltage and thesecond voltage, and the ratio N of the first and second currents. For aratio N of 2, a change in temperature T of 1° C. relates to acorresponding change in ΔV_(BE) of 59.7 μV, whereas for N=10, thecorresponding change in ΔV_(BE) for each degree of temperature Tincreases to 198 μV, or about 0.2 mV. Block 508 may be performedmanually by an operator, or may be performed by a processor, which maybe electrically connected to the voltage meter 120.

A test process 60 using the thermal sensor 100 in accordance withvarious embodiments of the present disclosure is shown in FIG. 6. Thetest process 60 may be run in test equipment 70 shown in FIG. 7. Thetest equipment 70 may be separate from the current source 110 and thevoltage meter 120. In some embodiments, either or both of the currentsource 110 and the voltage meter 120 are included in the test equipment70. An input/output (I/O) interface 720 may be electrically connected tothe current source 110 and the voltage meter 120. A processor 700 iselectrically connected to the I/O interface 720 and a memory 710, whichmay include volatile and non-volatile memory circuits.

When performing a temperature-dependent test procedure, thesemiconductor wafer 300 is heated (or cooled) to a temperature at whichperformance of the integrated circuit die 310 is to be characterized inblock 600. The temperature may be 0 degrees, 25 degrees, 50 degrees, 70degrees, 85 degrees, 120 degrees, or the like. The heating or coolingmay be performed in an oven held at a constant temperature, for example.An initial temperature reading may be taken by the test system 10 inblock 602. The initial temperature reading may be taken using theprocess 50 of FIG. 5, for example. The test system 10 may include thetest equipment 70, and operation of the test system 10 may be controlledby the processor 700, for example. In some embodiments, the currentsource 110 is controlled by the processor 700 through the I/O interface720. The processor 700 may access parameters for the process 50 from thememory 710. The parameters may include amplitudes of the first andsecond currents, for example. The block 602 is optional in someembodiments. The block 602 may be repeated multiple times prior toperforming the blocks 604 and 606. As an example, the processor 700 maystore voltage values read in the block 602 in the memory 710, andcompare a sequence of the voltage values to determine when thetemperature of the wafer 300 has stabilized (e.g., when the voltagevalues are substantially the same for a predetermined period of time ornumber of measurements).

After taking the initial temperature reading, and optionally when thetemperature of the wafer 300 has stabilized, circuit performance of theintegrated circuit die 310 is characterized in block 604 (e.g., byperforming a functional test on the die 310). In some embodiments, theblock 604 may test functions of the entire die 310, sub-circuits of thedie 310, or the like. A circuit test used to test the circuitperformance and/or functions of the die 310 may include analog test,radio frequency test, digital test, reliability test, and the like, andmay include a variety of test patterns and test parameters. Throughoutthe circuit test of block 604, the test system 10 may continuallymonitor the temperature of the semiconductor wafer 300 using the process50 of FIG. 5 in block 606. For example, if the circuit test requires 1minute to complete, the temperature of the wafer 300 may be taken about100 to 150 times. The temperature may be taken at multiple time pointsthroughout the circuit test of block 604, and the multiple time pointsmay be at regular and/or irregular time intervals. The temperaturereadings taken in block 606 may be stored in the memory 710, and may betimestamped and correlated to test readings of the circuit test.Immediately following completion of the circuit test in the block 604, afinal temperature reading of the semiconductor wafer 300 may be taken bythe test system 10 and stored in the memory 710 in block 608. The block608 is optional in some embodiments.

Based on the temperature and circuit performance (or pass/no pass,reliability performance) information collected by the test system 10 andthe test equipment 70 in the blocks 602, 604, 606, and 608, performanceof the integrated circuit die 310 may be characterized for eachtemperature at which the process 60 is performed in block 610. Thecharacterization may be correlated for each temperature measured in theblocks 602, 604, 606, and 608. In some embodiments, an averagetemperature may be calculated from the temperatures measured in theblocks 602, 604, 606, and 608, and the performance of the integratedcircuit and/or the entire integrated circuit die 310 may becharacterized for the average temperature. The temperature readingstaken in the process 60 may be calibrated as described above (e.g.,through a lookup table stored in the memory 710). The characterizationof the die 310 is rapid and accurate due to the use of the thermalsensor 100 of the test system 10 and the test equipment 70. The process60 may also be fully automated, which increases throughput and reduceshuman error.

Although the Figures have been described with reference to a single die,such as the die 310, other embodiments in which a stacked die is testedusing the thermal sensor 100 are also contemplated herein. The thermalsensor 100 may be integrated into a top die of the stacked die, forexample. In some embodiments, the thermal sensor 100 may be integratedinto an intermediate or bottom die of the stacked die, and accessible bythe test system 10 through a combination of solder balls, bumps,contacts, redistribution layers, metal layers, through-substrate vias(TSVs), and the like.

The test system 10, the test equipment 70, and the processes 50 and 60have many advantages. By arranging thermal sensors near specific,temperature-sensitive regions of integrated circuit dies, temperaturedata is better correlated to actual temperature at the site of thecircuit test being performed. The thermal sensor 100 is also faster andmore accurate than a thermal couple, not to mention being smaller andcapable of flexible integration into the die 310 and/or the scribe lines331-334 surrounding the die 310. The thermal sensor 100 may also bemonitored constantly throughout the circuit test, which provides moreand better temperature information for calibrating and/or characterizingcircuit performance of the die 310.

In accordance with various embodiments of the present disclosure, amethod includes measuring a first voltage across a test diode on asemiconductor wafer while injecting a first current into the test diode,measuring a second voltage across the test diode while injecting asecond current into the test diode, and determining temperature of aregion proximate the test diode according to difference between thefirst voltage and the second voltage.

In accordance with various embodiments of the present disclosure, amethod includes heating a semiconductor wafer in an environment at atemperature, determining temperature of the semiconductor wafer by athermal sensor circuit integrated in the semiconductor wafer, performinga circuit test on an integrated circuit of the semiconductor wafer, andcorrelating performance of the integrated circuit with the temperaturedetermined by the thermal sensor.

In accordance with various embodiments of the present disclosure, asemiconductor wafer comprises an integrated circuit die, a test diodeintegrated into the semiconductor wafer, a first current contactelectrically connected to an anode terminal of the test diode, a secondcurrent contact electrically connected to a cathode terminal of the testdiode, a first voltage contact electrically connected to the anodeterminal of the test diode, and a second voltage contact electricallyconnected to the cathode terminal of the test diode.

Although the present embodiments and their advantages have beendescribed in detail, it should be understood that various changes,substitutions, and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims. Moreover, the scope of the present application is not intendedto be limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods, and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

What is claimed is:
 1. A method comprising: heating a semiconductorwafer to a first temperature; determining the first temperature of thesemiconductor wafer by a thermal sensor circuit integrated in thesemiconductor wafer; performing a first reliability test on anintegrated circuit of the semiconductor wafer while the semiconductorwafer is at the first temperature; heating the semiconductor wafer to asecond temperature, the second temperature being different than thefirst temperature; determining the second temperature of thesemiconductor wafer by the thermal sensor circuit; performing a secondreliability test on the integrated circuit of the semiconductor waferwhile the semiconductor wafer is at the second temperature; andcorrelating a result of the first reliability test and a result of thesecond reliability test of the integrated circuit with the firsttemperature and the second temperature determined by the thermal sensorcircuit.
 2. The method of claim 1, wherein determining the firsttemperature is performed prior to the performing the first reliabilitytest.
 3. The method claim 1, wherein determining the first temperatureis performed during the performing the first reliability test.
 4. Themethod of claim 1, wherein determining the first temperature isdetermining the first temperature of the semiconductor wafer at multipletime points throughout the performing the first reliability test.
 5. Themethod of claim 1, wherein determining the first temperature of thesemiconductor wafer by the thermal sensor circuit integrated in thesemiconductor wafer comprises: measuring a first voltage across a testdiode while injecting a first current into the test diode, wherein thethermal sensor circuit comprises the test diode; measuring a secondvoltage across the test diode while injecting a second current into thetest diode; and determining the first temperature according to adifference between the first voltage and the second voltage.
 6. Themethod of claim 5, wherein determining the second temperature of thesemiconductor wafer by the thermal sensor circuit integrated comprises:measuring a third voltage across a test diode while injecting a thirdcurrent into the test diode; measuring a fourth voltage across the testdiode while injecting a fourth current into the test diode; anddetermining the second temperature according to a difference between thethird voltage and the fourth voltage.
 7. A method comprising: heating asemiconductor wafer, the semiconductor wafer including a thermal sensor;measuring a first voltage across a test diode of the thermal sensorwhile injecting a first current into the test diode; measuring a secondvoltage across the test diode while injecting a second current into thetest diode; and determining a first temperature of a region proximatethe test diode according to difference between the first voltage and thesecond voltage; performing a circuit test on an integrated circuit ofthe semiconductor wafer; measuring a third voltage across the test diodeon the semiconductor wafer while injecting a third current into the testdiode; measuring a fourth voltage across the test diode while injectinga fourth current into the test diode; determining a second temperatureof the region proximate the test diode according to difference betweenthe third voltage and the fourth voltage; and correlating performance ofthe integrated circuit with the first temperature determined by thethermal sensor, the second temperature determined by the thermal sensor,or both the first temperature and the second temperature.
 8. The methodof claim 7, wherein measuring the first voltage is measuring the firstvoltage across a test diode located on an integrated circuit die of thesemiconductor wafer while injecting the first current into the testdiode.
 9. The method of claim 7, wherein measuring the first voltage ismeasuring the first voltage across the test diode located on a scribeline of the semiconductor wafer while injecting the first current intothe test diode.
 10. The method of claim 7, wherein measuring the firstvoltage is measuring the first voltage across the test diode locatedproximate to a temperature-sensitive circuit of an integrated circuitdie of the semiconductor wafer while injecting the first current intothe test diode.
 11. The method of claim 7, wherein measuring the firstvoltage is measuring the first voltage across a PN junction in thesemiconductor wafer while injecting the first current into the PNjunction.
 12. The method of claim 7, wherein measuring the first voltageis measuring the first voltage across a diode-connected bipolar junctiontransistor in the semiconductor wafer while injecting the first currentinto the diode-connected bipolar junction transistor.
 13. The method ofclaim 7, wherein determining the first temperature is determining atemperature of the region proximate the test diode according to thedifference between the first voltage and the second voltage andaccording to a calibration lookup table.
 14. The method of claim 7,wherein measuring the second voltage is measuring the second voltageacross the test diode while injecting the second current havingmagnitude greater than magnitude of the first current into the testdiode.
 15. The method of claim 7, wherein measuring the second voltageis measuring the second voltage across the test diode while injectingthe second current having magnitude less than magnitude of the firstcurrent into the test diode.
 16. A method comprising: heating asemiconductor wafer having an integrated circuit formed therein; takingan initial temperature reading of a region of the semiconductor waferusing a test system including a test diode formed on the semiconductorwafer, wherein taking the initial temperature reading comprises:measuring a first voltage across the test diode while injecting a firstcurrent into the test diode, wherein the test diode is proximate to theregion of the semiconductor wafer; measuring a second voltage across thetest diode while injecting a second current into the test diode; anddetermining the initial temperature reading of the region according to adifference between the first voltage and the second voltage; performinga circuit test on the integrated circuit to determine performance of theintegrated circuit; taking a final temperature reading of the region ofthe semiconductor wafer using the test system; and correlating theperformance of the integrated circuit with the initial temperaturereading and the final temperature reading.
 17. The method of claim 16,further comprising monitoring the temperature of the region of thesemiconductor wafer during the step of performing a circuit test. 18.The method of claim 16, wherein the test diode comprises adiode-connected bipolar junction transistor.
 19. The method of claim 16,wherein the step of correlating performance includes correlatingperformance of the integrated circuit with an average of temperaturesdetermined by the test system at multiple time points.
 20. The method ofclaim 16, wherein taking the final temperature reading of the region ofthe semiconductor wafer using the test system comprises: measuring athird voltage across the test diode while injecting a third current intothe test diode; measuring a fourth voltage across the test diode whileinjecting a fourth current into the test diode; and determining thefinal temperature reading of the region according to a differencebetween the third voltage and the fourth voltage.